The invention relates to a dispersing device, particularly a ball or bead mill for use as a submersible mill, comprising a container that receives and processes a product to be dispersed, a grinding device with a housing that contains grinding bodies, where said housing has openings enabling the product to be dispersed to pass through, an agitating tool arranged in the housing and a first flow-producing device, where the housing and the agitating tool can move relative to one another and at least one shaft protrudes into the housing via a through-opening that allows the product to be dispersed to enter the housing.
A device of this kind disperses fine to very fine, solid particulate constituents in the liquid phase.
Three sub-steps occur simultaneously during the dispersion process:
1. Wetting of the surface of the solid material to be incorporated by the liquid constituents of the product to be dispersed,
2. Mechanical separation of agglomerates into smaller agglomerates and primary particles, and
3. Stabilization of primary particles, agglomerates and aggregates to prevent renewed clumping (=flocculation).
Although the following description primarily relates to the dispersion of paints and coatings, this processing technique can also be applied in a similar manner in other fields (e.g. biology, food processing technology, pharmacy, agrochemistry, ceramics industry and the like).
A grinding device of this kind is known from U.S. Pat. No. 5,194,783. This patent discloses an agitating submersible mill that disperses according to the circulation process. It essentially consists of a wear-resistant basket filled with grinding media designed as grinding balls, which is submerged in a double-walled container. A cylindrical drive shaft runs through the center of the basket. This drive shaft drives the bar-type agitator mounted inside the basket. The walls of the basket exhibit sieve-like perforations.
When dispersing paints, for example, it is of economic interest to minimize the use of relatively expensive primary colorant particles. The better the dispersion is, the more intense the color effect and gloss are. Thus, good dispersion can, for example, reduce the use of expensive primary colorant particles by permitting the use of cheaper secondary particles. In the ideal situation, each primary particle is wetted separately.
In order to enable circulation of the product to be dispersed through the basket, the drive shaft drives a flow-producing device in addition to the agitator. This flow-producing device must be positioned outside the basket in order to ensure adequate flow. Thus, the drive shaft penetrates the basket. A separating and sealing system is fitted at the point of penetration to prevent the grinding balls from escaping from the basket. The central position of the flow-producing device has definite advantages in terms of fluid mechanics, because it ensures uniform circulation throughout the container.
However, in order to carry out an economical dispersion process using the dispersing device known from the prior art, the product to be dispersed must be pre-dispersed. Pre-dispersion is preferably performed using a dissolver disk due to the fact that optimum pre-dispersion is indispensable from an economic standpoint, particularly in the case of agglomerates that are difficult to disperse and require the use of the grinding device during subsequent processing. An inadequately pre-dispersed product not only necessitates longer running times of the grinding devices known from the prior art, but it also frequently happens that the desired fineness is not attained. As a rule, omissions or errors in pre-dispersion cannot be compensated for by other systems, particularly because inadequately pre-dispersed products clog the holes in the basket during subsequent use of the grinding device, this hindering, or even completely stopping, circulation through the basket.
Although very satisfactory grinding results are achieved with the devices known from the prior art, they arexe2x80x94like virtually all agitating ball millsxe2x80x94subject to the disadvantage that the point where the shaft penetrates the grinding basket is provided with a dynamic friction gap or other similar means, through which the grinding balls can escape from the housing or the grinding basket into the container. Furthermore, dynamic friction gaps require relatively narrow tolerances in order to function properly, meaning that their manufacture is complex and expensive. However, even with the highest precision manufacturing and faultless operation, the problem still occurs that the grinding balls are unintentionally crushed in the friction gap, thereby destroying the friction gap and contaminating the product to be dispersed.
Another problem occurs at the annular through-opening between the shaft and the housing. During operation, these components move relative to one another. The product to be dispersed flows through this annular gap into the housing, as is required for the grinding process.
On the other hand, this through-opening is associated with the considerable disadvantage that grinding balls uncontrollably and unintentionally escape from the housing during the grinding process. This is disadvantageous in two respects. On the one hand, the product to be dispersed must be filtered again after the grinding process and prior to further processing in order to filter out the beads, thus necessitating an additional, time-consuming processing step. On the other hand, the loss of beads must be compensated for at regular intervals, as the grinding performance would otherwise decline.
A basket mill is known from EP 0 546 715, whose upper housing cover has a cylindrical collar on top, in which an impeller provided on the shaft runs. This device also does not prevent the grinding media from unintentionally leaving the housing.
Consequently, the technical object of the invention is to further develop a dispersing device of the kind specified at the outset, such that the grinding balls are reliably prevented from escaping from the housing or the grinding basket.
According to the invention, the object is solved in that a second flow-producing device, designed as an impeller, is provided in the region of the through-opening between the shaft and the housing, and in that the housing in the region of the through-opening is designed as a pump housing in the shape of a half-shell to accommodate the impeller, in order to prevent the escape of the grinding balls.
As a result of the design according to the invention, the relative movement generates a flow into the inside of the housing of the grinding device. This flow is so strong that it reliably prevents the grinding balls from escaping through the through-opening and into the product to be dispersed inside the container. In addition, the second flow-producing device results in more thorough mixing and draws the product to be dispersed into the housing of the grinding device more rapidly, thus increasing the throughput.
Finally, the flow-producing device deflects the grinding balls that escape from the grinding basket through the through-opening. Should this deflection be inadequate for preventing the grinding balls from getting into the through-opening in any manner whatsoever, the flow generated by the flow-producing device is sufficient to suck any balls that still escape back into the grinding basket or grinding device. Moreover, the design of the grinding device according to the invention is associated with the special advantage that the flow generated by the flow-producing device holds even considerably lighter and cheaper grinding balls (e.g. designed as glass beads) inside the grinding device, so that heavier and more expensive grinding balls, such as those made of zirconium oxide, which are usually required for numerous grinding processes due to their greater density and resultant weight, can be dispensed with. The use of cheaper grinding balls made of glass substantially reduces grinding costs.
The flow-producing device is preferably designed as an impeller and the housing of the grinding device has a pump housing in the region of the through-opening in order to accommodate the impeller. Thus, the impeller runs in an area of the housing of the grinding device specifically designed for its accommodation. The impeller and the housing thus form a pump for drawing liquid into the inside of the housing.
It is particularly advantageous if the pump housing is essentially in the shape of a half-shell, into which the impeller can be inserted such that its blades or webs face the housing wall.
In a particularly advantageous configuration, the upper region of the housing is of funnel-shaped design and provided on the side of its base facing away from the funnel with the pump housing for the impeller. In this context, the through-opening can be located at the base of the funnel, so that the product to be dispersed, which is drawn into the inside of the container by the impeller, flows down, or is drawn down the funnel walls.
The impeller can achieve greater pumping power if the pump housing has inclined walls that are adapted to the incline of the blades on the impeller.
The gap between the impeller and the housing is preferably designed such that it is larger than the diameter of the grinding balls, so that it is impossible for the grinding balls to get caught between the impeller and the pump housing.
In an advantageous configuration, the outer circumferential edge of the opening of the pump housing facing the inside of the housing is provided with a circumferential lip. This lip prevents the grinding balls from directly entering the region between the impeller and the pump housing. If the lip is of suitable design, the grinding balls are deflected back into the inside of the grinding device.
It has proven to be particularly advantageous for the impeller to be provided with a disk-shaped plate, on which at least one blade-like web is arranged, which extends essentially at an angle to the plane of the plate in the direction of the rotational axis of the plate and, in the radial direction on the plate, runs essentially at an angle to a tangent to the outer circumferential edge of the plate. In this context, it is possible to provide only one blade-like web on the plate, which extends in helical fashion from the central rotational axis of the plate on the outer circumferential edge, or also several wings. The second configuration has proven to be particularly effective in practice. In a simplified configuration, the webs are only arranged radially on the plate (without curvature and not offset at an angle).
In another configuration, the longitudinal extension of the webs is of sickle-shaped design, due to the fact that this design ensures better deflection of the beads and greater pumping power.
It is generally conceivable to have various web forms for the impeller, which can be exchanged depending on the viscosity of the medium to be ground. To this end, it is particularly advantageous when the impeller is mounted in the grinding device in detachable fashion, in order to ensure easy exchange. In this context, the impeller can be driven by the shaft that drives the flow-producing device, or also by another, hollow shaft that is concentric to the first shaft and surrounds it.
It has proven to be particularly advantageous for the agitating tool to have an annular disk that is provided with a circumferential, step-like shoulder, on which the plate of the impeller can be mounted. The impellers can then be easily exchanged, depending on the type of application. Alternatively, however, the impeller and the annular disk can also be designed as a single part.
As mentioned, the grinding device and the flow-producing device need not be driven by the same shaft. Rather, a second shaft can be provided which makes it possible to drive the agitating tool and the impeller connected to it separately from the first shaft.
It has proven to be particularly advantageous in practice for the second shaft to be a hollow shaft that is concentric to the first shaft and encloses it. In this configuration, the two shafts can be operated separately, so that pre-dispersion is performed by the flow-producing device. For fine dispersion, the outer hollow shaft engages the first shaft by way of a coupling and rotates at the same speed. Alternatively, it is also possible to drive the two shafts independently of one another and have them rotate at different speeds.
In a particularly advantageous configuration of the grinding device, the flow-producing device has means for dispersion. In addition, the grinding device is of adjustable height, where the grinding device can be submerged into the product to be dispersed and fully withdrawn again using the height adjustment feature, while the flow-producing device remains in the product to be dispersed. This design enables particularly simple and economical separation of pre-dispersion, which is performed by the flow-producing device preferably designed as a dissolver, and fine dispersion, which is performed by the grinding device.
Particularly good dispersion results are obtained, and the grinding device is especially easy to clean, if it is designed such that its housing has an open profile, the agitating tool is driven by a second shaft and connected to the second shaft by at least one connector running through the open profile, and the second shaft is a hollow shaft that encloses the first shaft.
The invention also applies to agitating ball mills in which the shaft is fixed and bears the agitating tool, and where the housing rotates relative to the agitator. In this case, an agitating ball mill of this kind is designed such that the second flow-producing device has a blade-like web, which is provided on the housing and generates a flow through the through-opening into the inside of the housing.
The flow-producing device is advantageously provided with one or more webs located on the housing in the region of the through-opening.